The spectroheliograph, as invented by George Ellery Hale in 1891, is an instrument which consists, essentially, of a prism between synchronously moving input and output slits. By means of, say, a telescopic lens, an image of an object to be studied is projected onto the plane of the moving input slit and an image is produced in turn on a photographic plate immediately beyond the moving output slit. As the input slit moves over the image projected by the telescopic lens, a picture is produced by the co-moving output slit at a narrow wavelength band determined by the positioning of the output slit relative to the input slit and the prism. In the spectrohelioscope, a modification of the spectroheliograph developed by Hale in 1924 but originally suggested by C. A. Young in 1870, an image for direct viewing is produced by rapidly oscillating the images of the slits, usually by means of two synchronously rotating prisms, so that the impression of a static picture is given to the eye.
Also known are image quality interference filters which are frequently employed in various applications to produce pictures in predetermined, narrow wavelength bands. Interference filters are of special interest in applications where the mechanical complexity of the spectroheliograph cannot be justified.
British Patent Specification No. 1,577,198 describes a system for microscopic analysis which utilises a monochromator with gratings. However, the object plane is imaged on the gratings and this is disadvantageous since imperfections, dust particles etc. on the gratings will show up in the final picture.